Add isLP64 to differentiate between LP64 and x86_64 targets.

is64bit has a meaning of ```version(X86_64)``` to the frontend, however gdc/ldc uses this to determine whether the target is 64bit.  Example:

object.d
```
struct TypeInfo_Struct
{
   /* ... */
   version (X86_64)
   {
      TypeInfo m_arg1;
      TypeInfo m_arg2;
   }
}
```
typinf.c
```
if (global.params.is64bit)
{
   /* emit m_arg1 and m_arg2 into toDt */
}
```

So on eg:  ARM64, is64bit is true, but X86_64 is false.

The alternative could be to introduce isX86_64 instead, and change everything else that uses is64bit as necessary.